The present invention relates to a method of determining the nominal driving behavior of a vehicle.
A method of the afore-described type is disclosed by DE 40 30 653 A1 describing a method for determining the slip angles and/or the lateral guiding forces of a decelerated vehicle. Based on a simplified vehicle model wherein the speed of the individual wheel, the steering angle, the yawing angle speed and the braking pressure are used as measurable variables, the slip angles and/or the lateral guiding forces are determined as estimated values. When plotting, in a diagram, the lateral guiding forces exerted on a wheel versus the momentary slip angles, a linear relationship will arise at least in respect of small slip angles. The slope of the straight line extending through the zero point is called "slip rigidity" of the respective wheel. However, a growing slip angle will render the relationship between lateral guiding force and slip angle non-linear. With a growing slip angle, the lateral guiding force approaches a peak value from which it negligibly departs again in the course of the curve. If the slip angles are in the non-linear range of the characteristic of lateral force/slip-angle, substantial differences occur between factual and simulated speeds of the yawing angle. As according to the prior known method the yawing angle speed is measured, the difference between measured and simulated yawing angle speeds are indicative of the transition from the linear to the non-linear range of the characteristic of lateral-force/slip-angle. Once it has been detected that the linear range of the lateral-force/slip-angle characteristic has been abandoned, the interconnection between lateral guiding force and slip angle is approximately defined by a straight line of a lower slope. For precisely adapting the vehicle model to the real conditions, in the prior art process, the slip rigidity of the front wheels and of the rear wheels is correspondingly modified so that the lateral-force/slip-angle characteristics of both axes conform to the real course of the curve.
Generally, a neutral driving conduct is striven for through a yawing moment control in a curve, which means that the self-steering gradient should, if possible, be zero. To that effect, it is easier for the driver to tackle a slight understeering by an additional steering lock than oversteering the vehicle. A neutral driving behavior exists if the slip rigidity values of the rear axle multiplied by the distance of the rear axle from the vehicle center correspond to the slip rigidity values of the front axle multiplied by the distance of the front axle from the vehicle center. If this product is smaller for the rear axle than the one for the front axle, thee is an oversteering behavior. The basic lay-out of modern vehicles, as a rule is slightly understeering. Assuming that the slip rigidities are identical at the front and rear, the vehicle model, in the linear range, always exhibits an understeering behavior if the distance of the rear axle from the vehicle center exceeds the distance from the front axle because the slip rigidities take constant values. However, if the slip rigidity values with a growing slip angle decrease, it might happen that the slip angles of the rear axle are already within a range in which the slip rigidity values are reduced while the front axle is still in the linear range of the characteristic of lateral-force/slip-angle. At that moment, the vehicle model would exhibit an oversteering behavior. This will involve danger, especially so if the vehicle model serves for computing a nominal value of, for example, the nominal speed of the yawing angle. In that case, the vehicle control would receive a nominal value corresponding to an oversteering behavior thus requiring a control manipulation causing the vehicle to oversteer. This involves great danger as it is substantially more difficult for the driver to tackle an oversteering than an understeering behavior. Even if the real vehicle with no manipulated control is caused to exhibit an oversteering behavior, the vehicle control, initially, does not interfere because such a behavior would then correspond to the nominal value.
The object of the present invention resides in providing a method for determining the nominal behavior of a vehicle which also takes into account the non-linear range of the lateral-force/slip-angle characteristic but preventing a nominal value corresponding to an oversteering behavior from occurring.